Patent Document 1 discloses a method of updating of a battery pack that updates firmware stored in a memory of a built-in control circuit in a rewritable manner, by an updating signal transmitted from a main body device that supplies power from a power source. In the updating method, when the memory of the battery pack is updated, an AC adapter is connected to the main body device, a charging and discharging stop signal for stopping discharging from a secondary battery of the battery pack to the main body device and charging of the secondary battery is transmitted to the control circuit of the battery pack through a communication line from the main body device in a state where power from a power source is supplied from the AC adapter to the main body device, and updating information is transmitted to the control circuit of the battery pack from the main body device in a state where the discharging to the main body device of the secondary battery and the charging from the main body device are stopped using the charging and discharging stop signal, thereby updating the memory.